The present invention relates to the effective alkylation of isopentane to reduce the Reid vapor pressure (RVP) of the resulting alkylate, and more particularly to the alkylation of isopentane in combination with isobutane to reduce alkylate RVP.
Isopentane, a product of many refinery processes, including alkylation, is a major contributor to refinery gasoline pool RVP because of its high RVP of 20.5 pounds per square inch (PSI). Alkylation of isopentane is one way to reduce its RVP. However, alkylation of pure isopentane or suppression of isopentane from amylene alkylation results in low C5+ yields, low octanes and low isopentane conversion.
Classic studies of alkylation show that isopentane reacts readily to form alkylates that are generally one carbon number larger than isobutane alkylates and slightly lower in octane. Historically, however, isopentane has not been alkylated because of its high value as a gasoline blendstock, relative to isobutane, and the lower value of isopentane alkylates relative to isobutane alkylates.
In fact, isopentane alkylation has been so unfavorable economically in the non-reformulated gasoline technologies that studies of the concurrent alkylation of isobutane and isopentane have not been published, only studies in which single isoparaffins were fed. This is important because past work has overlooked possible synergies resulting from interaction between isobutane and isopentane alkylation.
It has been discovered that the addition of small amounts of impure isopentane, with isopentane:olefin ratios on the order of 0.1 to 2.0, to conventional isobutane alkylation, with isobutane:olefin ratios on the order of above 5, of mixed butylene and amylene olefins gives superior C5+ yields, octanes, and isopentane conversion to other methods of isopentane conversion, on the order of 16 to 40 percent.
In a general sense, the invention comprises a method of alkylating a mixture comprising olefins having from about 3 to 5 carbon atoms per molecule (C3 to C5 olefins) by mixtures of isobutane and isopentane with an acid catalyst to form a high octane alkylate that has relatively low Reid vapor pressure, comprising the steps of: charging a mixed C3 to C5 olefin stream with an isobutane stream to form an isobutane-charged alkylation feed; charging said isobutane-charged alkylation feed with an isopentane stream to form a mixed isoparaffin-charged alkylation feed; and reacting said mixed isoparaffin-charged alkylation feed with an acid catalyst to form an alkylate that has high octane combined with low Reid vapor pressure. The invention also comprises apparatus for implementing this process.